Owing to their functionality, α-hydroxy ketones are important synthetic units for a multitude of chemicals, for example:                for the preparation of heterocycles such as imidazoles (EP-A 252162) and imidazolones (Journal of the Chemical Society Perkin II 1981, 310), the precursors for active medicament and crop protectant ingredients,        as a reducing agent in dyeing for the dyeing of textiles owing to their reduction capacity (EP-A 364752),        as an aroma in foods, such as acetoin or the diacetyl resulting therefrom,        and they are additionally important as a structural motif and frequent constituent in natural products, which might be of great significance for future medicaments (Journal of the American Chemical Society, 2004, 3070).        
α-Keto acids can be prepared from α-hydroxy ketones of the general formula I where R′═R═H by a suitable oxidation of the alcohol functionality. The uses of α-keto acids include those as pharmaceutical products and precursors.
In addition, it would be possible to prepare the keto acid precursors of the important methionine or methionine hydroxy analogue (MHA) products by a suitable oxidation of the α-hydroxy ketones of the general formula I where R′═R═H and R″═CH3S(CH2)2. The crucial advantage would be that the highly toxic and dangerous hydrocyanic acid (HCN) used to date could be replaced by the much less dangerous formaldehyde (HCHO).
It is known to those skilled in the art that α-hydroxy ketones of the general formula I
can be prepared in various ways, for example by:                1. Benzoin reaction: this is understood to mean the addition of two aldehydes to an α-hydroxy ketone by an umpolung of an aldehyde with cyanide as a catalyst. Owing to the stabilization by the aromatic ring, the reaction with cyanide is restricted to aromatic aldehydes (Organische Chemie [Organic Chemistry], K. Peter C. Vollhardt VCH, 1. Auflage, 1988 p. 1025, and also Castells et al. Tetrahedron Letters 1985, 26, 5457). Thiazolium carbene-catalysed benzoin condensation has also been known for some time (Breslow, Journal of the American Chemical Society 1959, 3719).        2. Stetter reaction: this is understood to mean the addition of a reversed-polarity aldehyde with a 1,4-electrophile, which can be catalysed with carbene.        
Known carbene catalysts are various N-heterocyclic carbenes which are useable in various ways—for example as ligands for transition metals, as nucleophilic catalysts for acylations, transesterifications or ring-opening polymerizations. The carbene catalyst classes used for umpolung of aldehydes are the thiazolium carbenes known from nature, the imidazolium carbenes and the triazolium carbenes.
The active carbene catalysts are normally sensitive towards water and air. They are obtained from the corresponding imidazolium, thiazolium or triazolium salt by deprotonation with a base. The bases used include sodium hydride, potassium hydride or potassium tert-butoxide in THF (Nair et al., Angewandte Chemie, 2004, Vol. 116, 5240 ff.).
However, it is also known that catalytic amounts of the active catalysts can be obtained in situ by the use of a biphasic system (Waymouth et al., Journal of the American Chemical Society, 2003, 3046).                3. Carbene-carboxylate/CO2 adducts: the CO2 adducts of the imidazolium or imidazolinium carbenes 1-7 which are listed in Table 1 are known.        
TABLE 1Known CO2 adducts of imidazolinium and imidazoliumsaltsStructure(Compound No.)Name Lit.1,3-Diphenylimidazolinium-2-carboxylateW. Schössler, M. Regitz.Chem. Ber. 1974, 107, 1931-1948. 1,3-Diisopropyl-4,5-dimethyl-1H-imidazol-3-ium-2-carboxylateN. Kuhn, T. Kratz.Synthesis 1993, 561-562.N. Kuhn, M. Steimann, G.Weyers. Z. Naturforsch.1999, 54b, 427-433. 1,3-Di-tert-butylimidazolium-2-carboxylateK. Ishiguro, K.Hirabayashi, T. Nojima, Y.Sawaki. Chem. Lett. 2002,796-797. 1,3-Dimethylimidazolium-2-carboxylateJ. D. Holbrey, W. M.Reichert, I. Tkatchenko,E. Bouajila, O. Walter, I.Tommasi, R. D. Rogers.Chem. Comm., 2003, 28-29. 1,3-Dimesityl-1H-imidazol-3-ium-2-carboxylateH. A. Duong, T. N.Tekavec, A. M. Arif, J.Louie. Chem. Comm., 2004,112-113. 1,3-bis (2,6-Diisopropylphenyl)-1H-imidazol-3-ium-2-carboxylate 3-Tert-butyl-1-methyl-1H-imidazol-3-ium-2-carboxylateI. Tommasi, P. Sorrentino.Tetrahedron Letters, 46(2005) 2141-2145.
Compounds 5 and 6 have been used for the preparation of oligomeric and polymeric isocyanates, especially uretdiones and isocyanurates (WO 2005-113626). Further carboxylate adducts of the imidazolium and imidazolinium structure and possible catalytic uses are not known to date.
The disadvantages in the prior art of acyloin formation from aldehydes are as follows:                The use of bases in acyloin formation reactions, for example for the release of catalytically active imidazolium carbene, causes side reactions such as aldol condensation between aldehydes and        leads to a reduction in the yield of desired α-hydroxy ketones (acyloin).        The bases used subsequently have to be removed from the product, which means additional work and can lead to difficulties in the achievement of high purities—as required, for example, for pharmaceutical products.        In addition, no universal synthesis scheme for crossed aldehyde-aldehyde additions is known to date, in which different aldehydes are reacted with one another (cf. Angewandte Chemie, 2004, 1348).        In order to use imidazolium carbene carboxylates II or imidazolinium carboxylates III as the source of the actually active imidazolium or imidazolinium carbene catalysts, a decarboxylation is first necessary.        The decarboxylation of such, for example, unsaturated, imidazolium carbene carboxylates proceeds, according to H. A. Duong et al. (cf. above), however, only at temperatures from 187° C., so that the release of the catalytically active carbene was to be expected only at such temperatures. This result would have dissuaded the person skilled in the art from seriously considering the imidazolium carbene carboxylates as catalysts in acyloin formation owing to the marked side reactions at such high temperatures in the acyloin formation reaction.        